1. Field of the Invention
The present invention relates to a thermal printer for printing characters etc., and more particularly, to a thermal printer that uses a power source such as a cell in which at least its internal resistance varies due to a temperature change.
2. Description of the Related Art
Heretofore, such a thermal printer has been used for printing characters etc., and is provided with a plurality of heater elements arranged in a thermal head for effecting a print operation by thermic color development or thermal transfer by supplying an energizing pulse signal in accordance with a print signal to the heater element.
In a thermal printer that uses a cell as a power source, a voltage drop is caused upon printing due to internal resistance of the cell when driving the thermal printer. Therefore, the above each heater element cannot be driven with a proper energy, and unevenness in the density of printing etc. may be caused.
To solve the above-mentioned problem, the conventional thermal printer is provided with a dummy load generating circuit. Accordingly, when the thermal printer is powered on, current is supplied to the cell by using the above dummy load generating circuit, and the internal resistance of the cell is calculated by detecting its voltage drop, and in consideration of the internal resistance, an energizing pulse width for driving each heater element is calculated as disclosed in Japanese Patent Application Laid-open No. Hei 6-115143.
In the above-mentioned conventional thermal printer, a dummy load circuit for calculating the internal resistance of a cell is required. Therefore, there arise problems in that with the complication of the configuration thereof, the price of the printer becomes high, and useless power is consumed in the dummy load circuit.
Further, since the calculation of the internal resistance is only carried out when power is turned on, change in the internal resistance due to temperature changes of a cell during printing is not taken into consideration. As a result, there occurs a case where energy for driving a heater element is not a proper value, which causes a problem in that the unevenness of printing density is generated.
In addition, other than the case where a cell is used, if a power source is used in which the internal resistance varies due to temperature changes, a similar problem occurs.
In view of the above-mentioned problems, the present invention has been made, and an object of the invention is to provide a printer in which unevenness of printing quality due to temperature changes of a power source can be prevented from occurring as well as can be prevented the useless power consumption of a dummy load with a simple configuration.
According to the present invention, there is provided a thermal printer, which is driven at least by a power supply in which the internal resistance varies depending upon a temperature, and is provided with a plurality of heater elements arranged in a thermal head for effecting a print operation by supplying an energizing pulse in accordance with a print signal applied to the heater element, characterized by comprising: power supply temperature measuring means for measuring a temperature of the power supply; storage means for storing a table showing a relationship between the temperature and the internal resistance of the power supply; and pulse width calculating means for acquiring, by referring to a table stored in ROM, a corresponding resistance on the basis of the temperature measured by the temperature measuring means, and for calculating a pulse width of an energizing pulse while the resistance is taken into consideration.
The pulse width calculating means acquires the corresponding resistance based upon the temperature measured by the temperature measuring means, by referring to a table stored in the storage means and calculates the pulse width of an energizing pulse for driving each heater element while taking the acquired resistance into consideration.
A cell can be used for the above power source.
Further, according to the present invention, a configuration may be employed in which a thermal printer further comprises power supply discriminating means that is driven by switching the cell and a DC power supply, acquired from an AC power supply, for discriminating by which of the cell or the DC power supply acquired from the AC power supply the thermal printer is driven, wherein the pulse width calculating means calculates the pulse width of the energizing pulse by referring to the table when the power supply discriminating means judges that the thermal printer is driven by the cell.
Further, a thermal printer according to the present invention may be configured such that the pulse width calculating means sets the resistance to a given fixed value, and calculates the pulse width of the energizing pulse when the power supply discriminating means judges that the thermal printer is driven by a DC power supply acquired from an AC power supply.
Furthermore, a thermal printer according to the present invention may be configured such that the pulse width calculating means calculates the pulse every time one line is printed.